There has recently developed a growing mood of informality in home and restaurant dining which has been reflected in the increased use of a wood, accompanied frequently with leather, decor. To augment the ambience created by that motif, dinnerware has been sought demonstrating an earthenware, stoneware, and/or ironstone appearance, and which exhibits high strength, excellent chemical durability, and low loss of toxic elements during use.
Lead-containing glazes have been employed for many years to decorate ceramic bodies with bright, glossy finishes. Most commonly, the glazes are fritted, mixed with a vehicle and, frequently, a binder and deflocculant, and the resultant slurry sprayed or otherwise applied to the surface of the ceramic body. But, to achieve the desired earthenware, stoneware, and/or ironstone appearance, it has been necessary to modify the bright, lead-containing glazes to impart a semi-gloss or even a matte finish thereto while developing a speckled brown hue therein. What has been deemed most desirable is a matte or semi-gloss finish in a warm, off-white color with flecks of a warm brown hue distributed throughout. Nevertheless, inasmuch as the dinnerware may be the object of further decoration, for example, the application of the logo of a restaurant thereon, the color and flecks cannot be so pronounced as to detract attention from that focal point.
Traditionally, the production of glazes demonstrating matte finishes has comprehended two processes: (1) underfiring the glazes; or (2) forming crystallites therein. Underfiring a glaze to produce a matte finish grossly increases the surface area thereof by preventing the frit powders from flowing together to form a non-porous glass over the body. This circumstance increases the susceptibility of the glaze to chemical attack and subsequent release of toxic elements such as lead. The formation of crystallites leads to a roughened surface and, more importantly, can tend to deleteriously alter the composition of the glaze. Hence, the development of crystallites may deplete the frit of a vital constituent, e.g., SiO.sub.2, as in the formation of calcium silicate, or it may cause the dilution of the glaze composition with a material which lowers the inherent chemical durability thereof, e.g., ZnO.
A speckled appearance has been imparted to glazes in the past via the incorporation of large (.about.150 microns) particles of Fe.sub.2 O.sub.3, MnO.sub.2, or "brown glass" which have been coated with an organic plastic to retain them in suspension in the frit slurry, the frit particles conventionally passing a No. 200 U.S. Standard Sieve (74 microns). The large particles are admixed with the glaze slurry at the conclusion of the mixing/melting operations so as to retain their dimensional integrity. The particles are relatively inert in the fused glaze. However, their large size precludes the development of a smooth glazed surface.